Instead, the bloom attracted a swarm of hungry copepods. The tiny crustaceans graze on phytoplankton, which keeps the carbon in the food chain and prevents it from being stored in the ocean sink. Researchers from the Alfred Wegener Institute for Polar and Marine Research reported that the copepods were in turn eaten by larger crustaceans called amphipods, which serve as food for squid and fin whales.

Advertisement

No diatoms

The grazing effect had not been seen in previous fertilisation experiments. These had caused blooms of diatoms, a type of phytoplankton that is protected against grazers by a hard shell of silica. But the Lohafex experiment did not trigger a diatom bloom because there was little silicic acid available in the water for diatoms to build their shells from.

Lohafex researchers say the results suggest that using iron fertilisation to increase the ocean carbon sink would rely on a complex chain of events, making it difficult to control. The Southern Ocean is thought to be the planet’s largest ocean carbon sink. But most of the northern half of the region is low on silicic acid, ruling it out as an option for carbon fertilisation.

The researchers tried to provoke a second bloom by fertilising the same patch of ocean three weeks later, with no success – most probably because the water was already saturated in iron.

“It seems that if it is possible to fertilise enough ocean to make a difference to climate, we would need to turn vast ocean ecosystems into giant plankton farms,” says Caldeira.

More experiments

A US commercial company called Climos has been planning a separate fertilisation trial for later this year.

“We look forward to seeing the Lohafex results in more detail, however this certainly does not change our plans to facilitate a larger scale experiment under more favourable nutrient conditions,” CEO Dan Whaley told New Scientist.

In previous interviews Whaley has said that Climos could one day seek to sell carbon offset credits similar to those offered by tree planting schemes. However, the International Maritime Organization‘s London Convention is in the process of drafting international regulation for ocean fertilisation activities. Early discussions suggest it is not in favour of commercial fertilisation schemes.